1. Field of the Invention
This invention relates to a resonant light deflector, or a resonant scanner, for use with optical beam scanning apparatus that are required to perform precise optical scanning, such as image recording or reading apparatuses for use in the making of printing plates. More particularly, this invention relates to a resonant scanner that is capable of reducing the variations in resonant frequency due to the changes in the temperature of the scanner section during its operation.
2. Prior Art
An optical scanning apparatus in which a light beam deflected in the direction of main scanning performs two-dimensional scanning of an object being transported in the sub-scanning direction is used with various types of image recording and reading systems In this optical beam scanning apparatus, the light beam emitted from a light source such as a semiconductor laser is reflected and deflected in the direction of main scanning by means of a light deflector and the object being transported at a given speed in the sub-scanning direction which is generally perpendicular to the direction of main scanning is scanned two-dimensionally to perform image recording or reading.
While various types of light deflectors such as a polygonal mirror and a galvanometer mirror are employed in the optical beam scanning apparatus described above, a resonant light deflector which is generally referred to as a "resonant scanner" is commonly used with optical beam scanning apparatuses that are required to perform precise optical scanning, such as image recording or reading systems for use in the making of printing plates. In a resonant scanner, a reflecting mirror that deflects an incident light beam by reflection is allowed to oscillate by self-excitation at the inherent (resonant) frequency of a resonant drive system (a resonant drive mechanism) consisting of a movable section containing this reflecting mirror and a spring that elastically supports this movable section, and predetermined optical scanning is accomplished by controlling the resonant drive system, particularly the amplitude of its oscillation. The resonance by self-excitation of this resonant drive system is maintained by permitting a current having the resonant frequency to flow through the drive coil on a drive motor.
The resonant scanner described above suffers from the disadvantage that the resonant frequency will change with temperature. As mentioned in the previous paragraph, the resonant scanner oscillates the reflecting mirror by bringing into resonance the resonant drive system consisting of the movable section and the spring. If the moment of inertia of the movable section (including the reflecting mirror) is written as J and the spring constant of the spring as k, the resonant frequency f is expressed by: ##EQU1## If the magnitude of the electromagnetic force imparted by the magnet and the coil is written as M and the attenuation constant as C (assuming a proportional attenuation), the amplitude of oscillation .theta..sub.f of the resonant drive system is resonance is expressed by: ##EQU2## The resonant frequency and the amplitude of oscillation are both largely dependent on the spring constant of the spring and the magnetic flux passing through the magnet and the coil. However, as is well known, not only the spring constant but also the magnetic flux through the magnet and the coil will vary with temperature. Accordingly, the resonant frequency of the reflecting mirror will vary with the temperature of the scanner section. The principal reasons for such temperature-dependent variations of resonant frequency are the changes that occur in the spring characteristics of the leaf spring and the magnetic flux through the magnet on account of temperature variations and, further, the resulting changes in the amplitude of oscillation of the drive mechanism (including the reflecting mirror). Thus, the resonant frequency of a conventional scanner has temperature dependency which, when expressed by (.DELTA.f/f)T, is typically on the order of -2.times.10.sup.-4 /.degree.C.
To perform optical scanning, for example, image reading or writing (exposure) with an optical beam scanning apparatus using a resonant scanner having such temperature dependency, cycles of the following operation are repeated: the resonant scanner is first driven for a while until the resonant frequency is stabilized and, thereafter, actual two-dimensional optical scanning is initiated and the scanner is brought to a rest after the scanning is completed. As shown in FIG. 6, if the resonant scanner starts to be driven, the temperature T of the scanner section, particularly of the drive coil section, rises first rapidly, then gradually afterwards whereas the resonant frequency f decreases first rapidly, then gradually afterwards. As a result, the deflection frequency is high in the initial period of optical scanning and decreases gradually toward the end of optical scanning.
Therefore, if the temperature of the scanner section in the optical beam scanning apparatus changes, the resonant frequency of the scanner, or the frequency of the cycles of main scanning with a light beam will also vary, causing an error in the image size in the sub-scanning direction on account of the change in the interval between scanning lines. For instance, in the recording of a plurality of images where images are successively written, the first output image will have a different dimension from the second output image in the sub-scanning direction. If the feed speed in the sub-scanning direction is written as v, the output dimension as L and the number of main scanning lines in the dimension L as y, the interval between main scanning lines, .DELTA.1, is expressed by v/f (1/f is the period of optical scanning with the scanner) and, hence, L=.DELTA.1.multidot.y=(v/f).multidot.y. Hence, the relationship between .DELTA.f, or the change in the resonant frequency f due to the change in the scanner section's temperature T by .DELTA.T, and .DELTA.L, or the change in the output dimension L in the sub scanning direction can be expressed by .DELTA.L/L=-.DELTA.f/f. If .DELTA.T is 5.degree. C. and L (the length of exposure in the sub-scanning direction) is 500 mm, .DELTA.L=L(-.DELTA.f/f)=L(2.times.10.sup.-4 .multidot..DELTA.T)=0.5 mm, and this presents a serious problem in application areas where high precision of optical scanning is required, particularly in the field of making color printing plates where image size is required to have an accuracy on the order of several tens of microns.
In order to solve this problem, it may be proposed that the temperature change that occurs in the coil section of the resonant scanner in an optical scanning mode and a non-scanning mode (see FIG. 6) when the resonant scanner is repeatedly driven and stopped should be eliminated by allowing the scanner to be continuously driven in the non-scanning mode. However, the life of the resonant scanner is determined by the sum of drive times for which the scanner is resonating, so driving the scanner continuously in a resonant state even in the non-scanning mode will shorten its life.
Instead of allowing the resonant scanner to be driven in the non-scanning mode as well as in the optical scanning mode, two improvements in the spring mechanism have been proposed. One improvement is characterized by the combined use of two springs having different temperature characteristics in the resonant scanner, the first spring having a constant that increases with temperature elevation and the second spring having a constant that decreases with temperature elevation. According to the other improvements, a magnetic spring is incorporated to compensate for the change in the spring constant of the spring used in the conventional resonant scanner due to temperature variation. However, the resonant scanner incorporating the first improvement suffers from the problem of low durability of springs; in addition, the combination of two springs having different temperature characteristics causes the movable part of the resonant drive system to oscillate in an imbalanced way, thus making it impossible to achieve correct optical beam scanning. The resonant scanner constructed according to the second improvement has the problem of complexity in mechanism. Furthermore, the magnetic spring, or a spring created by a magnetic circuit, has an unavoidable hysteresis loss which reduces the resonant oscillation of the mirror. To solve this problem, an increased current must be applied to the drive coil but then one great advantage of the resonant scanner, namely, its ability to cause the mirror to oscillate through large angles upon application of a small power, will be lost.